By recent development of digital recording technology and remarkable increase of data treated by a computer, the demand of magnetic tapes for storing these data has been greatly increased. Although mainly polyester films have been used as base films of such magnetic tapes, recently, requirements for making magnetic tapes thinner and enabling them to record at a higher density have been increased, and therefore, aromatic polyamide films having excellent thermal resistance, mechanical property and dimensional stability have been frequently used.
Further, as described above, the data recording density of a magnetic-recording medium has been remarkably increased accompanying with the recent development to have a high capacity, and small dimensional changes tend to become causes of data missing. Such dimensional changes are classified into an irreversible change such as heat shrinkage and a reversible change such as expansion/shrinkage due to temperature or humidity. Although it is desired that the irreversible change does not exist, it can be removed in a processing process by a treatment such as annealing. On the other hand, because the reversible change can not be removed easily, if temperature or humidity changes when the film is preserved, an expansion or a shrinkage of the film occurs, and because the positions of recorded data are shifted from proper positions to be set, it may become difficult to read the data.
In aromatic polyamide films, examples for controlling the above-described changes due to temperature and humidity are disclosed in the following patent documents 1 to 6. However, these documents are all supposed to be applied to use for circuit boards such as flexible print circuit board and film connectors, and because the films are used at a condition being laminated mainly with copper, the films are designed so that the thermal expansion coefficient of copper and that of the films are adapted to each other.
On the other hand, examples supposing use as supporting materials for magnetic-recording media are disclosed in the following patent documents 7 and 8. Document 7 intends to make a dimensional change small by controlling coefficient of humidity expansion, discloses the necessity that the coefficient of hygroscopic expansion β satisfies β≦100×10−6 (1/% RH), and discloses a film having β of from 10 to 60×10−6 (1/% RH) also in examples. However, it has been found that, if a magnetic layer and a backcoat layer are formed at a process for processing the film into a magnetic-recording medium, the dimensional change of the magnetic-recording medium and the dimensional change of the base film are different from each other by the influence of these layers. In such a case, the β of the magnetic-recording medium frequently increases as compared with that of the base film, and in order to minimize the dimensional change of the magnetic-recording medium, sometimes it is preferred to design the β of the base film to be minus. However, according to patent document 7, as described that “although the lower limit of β is not particularly restricted, about 1×10−6 is a limit for industrial production”(page 4, 18th paragraph), the concept and the technology for making the β minus are not disclosed. Further, in a case where the magnetic layer is formed by vapor deposition, it is necessary to flatten a curled portion generating after forming the magnetic layer (a phenomenon that a tape is curved at a condition where the magnetic surface is positioned inside) by heat treatment, and at that time, the properties of the magnetic-recording medium in its transverse direction greatly change, and there occurs a difference in property between the longitudinal direction and the transverse direction. Although it is necessary to control the expansion coefficient of the base film so as to make a difference between that in the longitudinal direction and that in the transverse direction in order that the dimensional changes in the longitudinal and transverse directions of the magnetic-recording medium coincide with each other, such a concept is not disclosed. Further, although the following patent document 8 defines thermal expansion coefficient in the thickness direction for the purpose of improving slitting property, similarly to patent document 7 it does not disclose the concept to control so that the dimensional changes in the longitudinal and transverse directions after processing into the magnetic-recording medium coincide with each other.
Further, although the following patent document 9 discloses an example defining a hygroscopic expansion coefficient of a magnetic tape with a supporting material of an aromatic polyamide film, also it does not disclose the concept to control so that the dimensional changes in the longitudinal and transverse directions coincide with each other.    Patent document 1: JP-A-2-84328    Patent document 2: JP-A-2-112935    Patent document 3: JP-A-3-60181    Patent document 4: JP-A-6-136156    Patent document 5: Japanese Patent 2,853,036    Patent document 6: Japanese Patent 2,952,907    Patent document 7: JP-A-8-297829    Patent document 8: JP-A-2003-160676    Patent document 9: JP-A-6-180836